This is an introductory astronomy survey class that covers our understanding of the physical universe and its major constituents, including planetary systems, stars, galaxies, black holes, quasars, larger structures, and the universe as a whole.

Taught By

S. George Djorgovski

Transcript

Well, then let's move to galaxy formation property, earliest galaxies that we can see. And, there is no natural division between galaxy formation and galaxy evolution, but we usually think galaxies are really high ages for a certain percent of the age of the galaxies and so on. And they form along with large scale structures in which they live, but they have extra complications in form of star formation, dissipation, and so on. And massive galaxies are done forming by and large, occasionally they have a major merger. But some of the dwarf galaxies may be just initiating star formation now. So galaxy formation is happening even now. Not very much of it. All right, so nowadays we are studying this at [INAUDIBLE] greater than about seven, which means the universe was one eighth of its present size and earlier, or roughly speaking, first billion years of the age of the universe. So this is what we think happens. Early on, there are no galaxies, there are no stars, there is dark matter, there are nonuniformities in it. Those grow under their own gravity, attract baryons, the gas, they become dark halos which then are containers inside of which normal stuff comes in. And then some point star formation ignites. And the ultraviolet radiation from those young starts ionizes the gas. Before then, gas is transparent to radio waves and infrared but not ultraviolet radiation because it's easily absorbed by hydrogen. After you ionize the hydrogen, it's transparent to UV light too. And that time is called the Reorganization Era. So this is sort of a cartoon version of it, you start with a Big Bang. There is micro background and there is what we call Dark Ages. There is nothing shining light in the universe. Dark [INAUDIBLE] slowly assembled gas into them. Star formation ignites and just removes all the neutral hydrogen, universe becomes transparent, and then this is what we observe as galaxy evolution from then on. So if you want to observe them, first thing you ask is where does the energy come from. And there are four potential sources of energy for young galaxies. First of all, you're collapsing larger clouds into something smaller. You have to get rid of the binding energy. That turns out to be of on the order of 10 to the 59 ergs for a typical galaxy. But the real energy is in nuclear burning in stars, it's an order of magnitude more, so integrated over whatever took. However you want to store information. It could be 10 to the 60th ergs for a typical galaxy. Now it's probably coincidence that the amount of binding energy that was released in converting interstellar clouds into stars is about the same as it took to assemble the galaxy, about 10 to the 59th ergs. And then if you have active nucleus Quasar black-hole, that can do anything. From nothing, all the way to about 10 to 60, 61 ergs, depending how luminous it gets. So there's plenty of energy. Some of the energy then ionizes the gas. Just like star-forming regions like Orion Nebula, and the gas emits in fluorescent lines. Three combination lines, Hydrogen, Oxygen, what have you. And by far the strongest of those will be Lyman alpha line, the first transition of neutral Hydrogen. Level II to Level I. And so that was seen as the primary tracer way to find star forming galaxies of high red shifts. Because high red shifts, that means it's now been red shifted into the visible part of the spectrum and so you can observe it with normal telescopes on the ground and you expect it to be pretty strong. So, there are two ways in which that was done. One is you take spectra of the sky. Occasionally, you run into one of those things. Like, that line, you just see the emission line and nothing else. Another way to look at them is to take pictures in narrow band filter that's centered on that line and compared with broader image, and so that's how it looks like. So this is field of some quasar and the top image on the right is just regular broadband red filter. The one underneath is picture taken in a narrow band filter that corresponds exactly to Lyman-alpha line. You can see everything else is suppressed. But this one galaxy really stands out. And that turns out to be emitting companion of that quasar at the same red shift. Both of those have been used extensively, mostly this kind of narrow-band imaging. This is one of the most distant, maybe still most distant confirmed galaxy with a spectrum, so a red shift of seven and this is done by a Japanese group in Subaru telescope. And you can see the pictures, there are two filters, there is nothing between those bars, and then you take narrow band image and boom, there is a galaxy shining that emission line. Another method which is now even more popular is so-called Lyman-Break Method. And that was first really done effectively by Chuck Sidel here. And it works like this. You have the young stars. There is interstellar medium, interhost galaxy, some neutral hydrogen. That hydrogen is going to absorb very effectively. All the light blue or the Lyman continuum break at 912 angstroms. If you look far enough there'll be even plenty of Lyman alpha force lines to take out everything blueward of Lyman alpha line. Spectral star-forming galaxies tend to be kind of flat. And then you're looking for objects that have a flat spectrum in red and then sudden drop in blue which is due to this absorption by the gas. And the example is shown there. This galaxy looks more or less the same in three-field visible visually infrared, and it's completely gone in the ultraviolet. And so this turns out to be a very efficient way of finding distant galaxies, which has now been pushed to the highest reaches that people actually can do. These are examples of some candidates for galaxies around red shift 8, 9, 10, 11 from Richard Ellis' work, and you can see that starting from the left there's nothing, then suddenly something shows up. And that's interpreted as one of those Lyman-breaks and that gives you the approximate red shift. This is not certain, we'll only know after we get real spectra with 30-meter telescope, but it's not too crazy idea. So this is what we can do today. And so this is the kind of observations that was then averaged to ask the question, what happens with star formation history at the highest radius we can probe, deepest images with Hubble. And that's those points to the right. So there is some uncertainty in those models, but the qualitative behavior is more or less what you expect.

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